Insulated power cables



United States Patent 3,429,983 INSULATED POWER CABLES Hermann Hofmeier,Dormagen, Germany, assignor to Farbenfabriken Bayer Aktiengesellschaft,Leverkusen, Germany, a German corporation No Drawing. Filed Nov. 23,1964, Ser. No. 413,325 Claims priority, application Germany, Nov. 27,1963,

US. Cl. 1'74-110 Int. Cl. H01b 3/ 42 This invention relates to insulatedcables, particularly to high voltage cables in which the conductor isinsulated with a polymeric material.

Heretofore electric cables have been manufactured with the conductor orconductors of the cable being covered by a layer of helically wrappedpaper tape impregnated with a hydrocarbon oil. Despite the advantageswhich paper possesses. as a convenient and inexpensive form of oilpermeable material with which to form the solid dielectric of an oilimpregnated high voltage cable, its use limits the maximum workingvoltage of such a cable for a given thickness of insulation.

Other materials, particularly polymeric materials, such as polyethylene,polyamides and polycarbonates have been employed as insulation forcables. However, they have one or more of the following disadvantages;namely, they do not have the necessary mechanical strength to withstandthe abrasion encountered in the manufacture of the cable and/or they areeasily attacked by insulating oils and moisture, Also, the di-electricconstants and the loss angles are high and are greatly influenced bytemperature within the operating range.

Therefore, it is an object of the present invention to provide a highvoltage cable surrounded with an insulating material having highmechanical strength. Another object of this invention is to provide ahigh voltage cable surrounded by an insulating material which isresistant to insulating oils and other insulating liquids. Anotherobject of this invention is to provide a high voltage cable surroundedwith an insulating material having a lower angle and a lower dielectricconstant than insulating materials heretofore employed. Still anotherobject of this invention is to provide a cable surrounded with aninsulating material wherein the electric breakup potential is extremelyhigh. Still another object of the present invention is to provide animproved high voltage cable capable of being operated at higher voltagesthan conventional insulated high voltage cables without correspondinglyincreasing the thickness of the insulation and hence the bulk of thecable.

To achieve these and other objects, the solid dielectric componentsurrounding the high voltage cable is formed from partially crystallizedpolycarbonates. It has been found that polycarbonate materials,particularly polycarbonate films which are partially crystallized andstretched, at least in the longitudinal direction, have all the physicalrequirements found to be necessary for insulation on high voltagecables. The partially crystallized polycarbonate films are formed from athermoplastic polycarbonate which has been stretched in a longitudinaldirection at least double its original length. It is preferred that thepolycarbonate have from about to 60 percent crystalline content and morepreferably from about to 50 percent crystalline content.

These partially crystallized polycarbonates, particularly polycarbonatefilms, have higher resistance to permanent heat in the stretch statethat the polycarbonates in the noncryst-alline state. Also, they havevery slight water absorption and are thus little permeable to watervapors. They have a high resistance to aging and are resistant toinsulating oils and similar liquids. Furthermore, films prepared frompartially crystallized polycarbonates have a 4 Claims lower loss angleand a lower dielectric constant than noncrystallized polycarbonatefilms.

In a preferred embodiment of the invention, a solid type singleconductor lead-sheathed cable composed of a plurality of strandedmetallic wires is surrounded throughout substantially its entire lengthby an insulating cover of a partially crystallized polycarbonatematerial. The partially crystallized polycarbonate material may beformed into a sheet and stretched in a longitudinal direction andhelically wrapped about the conductor. The film may be formed intostrips and applied preferably as a covering, with each turn of the stripsubstantially abutting, or with its edge spaced slightly from theadjoining edge of the adjacent turn rather than with overlapping of theadjoining turns.

It is also within the contemplation of this invention to from the soliddielectric component of the cable by wrapping about the conductor a filmformed of substantially impervious sheets of partially crystallizedpolycarbonates and having at least one side formed with a multiplicityto oil permeable channels extending thereacross from edge to edge. Theoil permeable channels which may be of virtually any cross-section, arepreferably formed at the same angle to the side edges of the film as theangle of lay at the film when it is helically wrapped about theconductor so that the channels in the cable insulation will extendlongitudinally of the cable. They may be formed in any convenientmanner, such as by cutting them into an originally ungroovedpolycarbonate sheet or by impressing them during fabrication of thesheet. If desired, the film may be roughened in order to facilitate theflow of oil between the layers of film to prevent the inclusion of air.A number of layers of the film are helically wrapped about a conductorto form an insulating covering thereon. These channels or roughenedareas will permit easy penetration of the insulating oil betweensuccessive layers of the wrapping and hence make possible effectiveimpregnation of the cable insulaion structure by the liquid dielectricmaterial. Alternating layers of smooth and embossed films may be used.

Any suitable crystallizable, high molecular weight polycarbonate may beused as insulating material, including most polycarbonates prepared inaccordance with the process disclosed in Canadian Patent 578,585.

The polycarbonates may be produced from a great number of dihydroxycompounds, that is, of aliphatic, cycloaliphatic and aromatic dihydroxycompounds. For example, there may be mentioned as aliphatic dihydroxycompounds, ethylene glycol, diethylene glycol, triethylene glycol,polyethylene glycol, thiodiglycol, ethylenethiodiglycol. The diandpolyglycols produced from propylene oxide-1, 2, ortho, meta, orparaxylylene glycol; propanedio1-l,3; butanediol-l,3; butanediol-l,4;Z-methylpropanediol-1,3; pentanediol-1,5; 2-ethylpropanediol-l,3;hexanedial-1,6; octanediol-1,8; l-ethylhexanediol 1,3, anddecanediol-l,l0.

As cycloaliphatic dihydroxy compounds are, for example,cyclohexanediol-1,4; cyclohexanediol-l,2; 2,2-(4-,4-dihydroxy-dicyclohexylene)propane and2,6-dihydroxydecahydronaphthalene. Examples of aromatic dihydroxycompounds are hydroquinone, resorcinol, 4,4'-dihydroxydiphenyl,2,2'-dihydroxydiphenyl, 1,4-dihydroxynaphthalene,1,6-dihydroxynaphthalene. Y

A preferred class of aromatic dihydroxy compounds which may be employedare the dimonohydroxyarylene sulphones and the dimonohydroxyarylenealkanes, such as,

4,4'-dihydroxydiphenylene sulphone, 2,2-dihydroxydiphenylene sulphone,3,3-dihydroxydiphenylene sulphone,4,4'-dihydroxy-2,2'-dimethyldiphenylene sulphone,

4,4-dihydroxy-3,3'-dimethyldiphenylene sulphone,2,2-dihydroxy-4,4'-dimethyldiphenylene sulphone,4,4-dihydroxy-2,2'-diethyldiphenylene sulphone,4,4-dihydroxy-3,3-diethyldiphenylene sulphone,4,4-dihydroxy-2,2'-di-tert. butyl-diphenylene sulp-hone,4,4-dihydroxy-3,3-di-tert. butyl-diphenylene sulphone,2,2-dihydroxy-1,1-dinaphthylene sulphone,4,4-dihydroxydiphenylenemethane, l,1-(4,4'dihydroxydiphenylene ethane,1,1-(4,4-dihydroxydiphenylene)-propane,1,1-(4,4-dihydroxydiphenylene)butane,1,1-(4,4'-dihydroxydiphenylene)-2-methyl-propane,1,l-(4,4-dihydroxydiphenylene) heptane,1,1-(4,4-dihydroxydiphenylene-l-phenylmethane,(4,4dihydroxydiphenylene)-(4methyl-phenylene) methane,(4,4-dihydroxydiphenylene)-(4-ethyl-phenylene) methane,(4,4-dihydroxydiphenylene)-(4-isopropyl-phenylene) methane,(4,4'-dihydroxydiphenylene)-(4-butyl-phenylene) methane,4,4'-dihydroxydiphenylene benzyl-methane,(4,4'-dihydroxydipheny1ene)-alpha-furyl-methane,2,2-(4,4-dihydroxydiphenylene)propane,2,2-(4,4-dihydroxydiphenylene)butane,2,2-(4,4-dihydroxydiphenylene)pentane (melting point 149-150 C.),2,2-(4,4-dihydroxydiphenylene)-4-methyl-pentane,2,2-(4,4-dihydroxydiphenylene)heptane (boiling point 198200 C. under 0.3mm. mercury gauge), 2,2-(4,4'-dihydroxydiphenylene)octane,2,2-(4,4-dihydroxydiphenylene)nonane (melting point 68 C.),1,1-(4,4'-dihydroxydiphenylene)-1-phenylethane,(4,4-dihydroxydiphenylene)-1-(alpha-furyl)ethane,3,3-(4,4'-dihydroxydiphenylene)pentane,4,4-(4,4-dihydroxydiphenylene)heptane,1,l-(4,4'-dihydroxydipheny1ene)cyclopentane,1,1-(4,4'-dihydroxydiphenylene)cyclohexane, 2,2-(4,4'-dihydroxydiphenylene) decahydronaphthalene,2,2-(4,4'-dihydroxy-3,3-dicyclohexyldiphenylene)- propane,2,2-(4,4-dihydroxy-3-methyldiphenylene)propane,2,2-(4,4-dihydroxy-3-isopropyl-diphenylene)butane,1,1-(4,4'-dihydroxy-3,3 '-dimethyldiphenylene) cyclohexane, 2, 2-4,4'-dihydroxy-3,3 '-dibutyldiphenylene) propane, 2,2-(4,4'-dihydroxy-3,3 '-diphenyldiphenylene propane,2,2-(4,4-dihydroxy-2,2'-dimethyldiphenylene) propane,1,1-(4,4'-dihydroxy-3,3-dimethyl-6,6'-dibutyldiphenylene)butane,1,1-(4,4-dihydroxy-3,3-dimethyl-6,6'-ditert. butyldiphenylene ethane,1,1-(4,4-dihydroxy-3,3-dimethyl-6,6-di-tert. butyldiphenylene)propane,1,1-(4,4'-dihydroxy-3,3'-dimethyl-6,6'-di-tert.butyldiphenylene)butane,- 1,1-(4,4-dihydroxy-3,3-dimethyl-6,6-di-tert.butyldiphenylenefisobutane,1,1-(4,4'-dihydroxy-3,3'-dimethyl-6,6'-di-tert. butyldiphenylene)heptane, 1,1-(4,4'dihydroxy-3,3'-dimethyl-6,6'-di-tert.butyldiphenylene)-I-phenyl-methane,1,1-(4,4dihydroxy-3,3-dimethyl-6,6'-di-tert.butyldiphenylene)-2-methyl-2-pentane,1,l-(4,4'-dihydroxy-3,3-dimethyl-6,6-di-tert(butyldiphenylene)-2ethyl-2-hexane, and1,1-(4,4'-dihydroxy)-3,3-dimethyl-6,6-di-tert. amyldiphenylene) butane.

Among the great number of suitable dimonohydroxy arylene alkanes the4,4'-dihydroxydiphenylene alkanes are preferred, especially the2,2-(4,4-dihydroxydiphenylene)- 4 propane and the1,l-(4,4-dihydroxydiphenylene)cyclohexane.

In some cases mixed polycarbonates prepared from at least two differentdihydroxy compounds, such as, at least one aromatic and at least onealiphatic dihydroxy compound yield films having desirable properties.

The polycarbonates may be prepared by several different processes, forexample, they may be prepared by the introduction of phosgene intosolutions of dihydroxy compounds or of mixtures of the aforesaiddihydroxy compounds in organic bases such as dimethylaniline, diethylaniline, trimethylamine and pyridine or in different organicsolvents such as petrol, ligroin, cyclohexane, methylcyclohexane,benzene, toluene, xylene, chloroform, methylene chloride, carbontetrachloride, trichloroethylene, dichloroethane, methyl acetate andethyl acetate, with addition of an acid binding agent, e.g., tertiaryamines. A process particularly suitable for producing polycarbonatesconsists in introducing phosgene into the aqueous solution or asuspension of alkali or alkaline earth metal salts, such as, lithium,sodium, potassium and calcium salts with a dihydroxy compound,preferably in the presence of an excess of a base such as lithium,sodium, potassium, calcium hydroxide or carbonate. The polycarbonatethen precipitates out from the aqueous solution.

The conversion in the aqueous solution is promoted by the addition ofinert solvents of the kind mentioned above which are capable ofdissolving phosgene and eventually the produced polycarbonate.

It is also possible to react bis-chlorocarbonates of di-hydroxycompounds, with the aforementioned dihydroxy compounds. The condensationproceeds suitably in the presence of inert solvents, and acid-bindingagents, e.g., tertiary amines.

In using phosgene or bis-chlorocarbonic acid esters as derivatives ofthe carbonic acid in producing polycarbonates, it may be advantageous touse catalysts. Such catalysts are, for example, tertiary or quaternaryorganic bases or salts thereof, such as trimethylamine, triethylamine,dimethylaniline, diethylaniline, dimethylcyclohexylamine, and pyridine,or for instance, the corresponding hydrochlorides in amounts from about0.05 to about 5 percent by weight.

In some cases, it may be preferable to add surface active agents, suchas, alkali metal salts of higher fatty acids or of sulphonic acids ofhigher aliphatic or of aromatic hydrocarbons and polyoxyethylatedalcohols and phenols.

The reaction conditions for producing the polycarbonates are notcritical, however, it is preferred that the phosgene react with thedihydroxyl compounds in a 1:1 mol ratio. Suitable temperatures are fromabout 0 C. to about 320 C.

Partially crystalline polycarbonate films may be prepared by anysuitable method known to the art per se. For example, the methoddisclosed in US. patent application Ser. No. 708,740, filed Jan. 1,1958, by Prietzschk et al., now abandoned. The polycarbonate may begissolved in an organic solvent, and cast therefrom as a It has beenfound that an improved result is obtained if the polycarbonate filmhaving a characteristic value of 1.67 as determined by the method ofPrietzschk, is prepared for stretching by casting it from apolycarbonate solution in a mixture of a good solvent for thepolycarbonate and a solvent which is a relatively poor and/ ornonsolvent for the polycarbonate. It is preferred that the poor ornon-solvent be less volatile than the solvent under the operatingconditions and have a relatively high boiling point. Polycarbonatearticles which are obtained from solutions employing only a good solventof low boiling point such as. for example, methylene chloride, have adegree of crystallization which corresponds to the characteristic valueof not higher than about 1.60, but their physical properties are notimproved by stretching. On

the contrary, when a mixture of good and relatively poor solution whichwas then stretched to about 3.5 times and/or nonsolvents are used with'apolycarbonate, the its unstretched length at a temperature of about 250crystallization" characteristics value of the product is C. A strip ofthe poly[2,2-bis(4-hydroxyphenyl)propane higher than about 1.67. Thestretched product has a carbonate] film having a crystalline content of40% was shape fiber diagram and improved physical characteristics. 5wrapped around the conductor of a high voltage cable. Any suitablesolvent mixture may be employed in forming the film, such as, forexample, a mixture of a good EXAMPLE 2 solvent such as methylenechloride, chloroform and the A solution of the polycarbonate from2,2-(4,4-dihylike, with a relatively poor and/or non-solvent suchas,droxydiphenyl)propane is cast to yield a film of a thickfor example,halogenated hydrocarbons including ethyl- 10 ness of about 0.1 mm. Thisfilm was stretched on a copper ene chloride and trichloroethylene,aromatic hydrocarbons bl k he ted to 170 C, with a velocity of about 0.5meter including benzene and toluene, aliphatic hydrocarbons per minute,the film being drawn over the corner of the including hexane andheptane; ketones including acetone, block.

methylethylketone, and dissopropylketone esters including The film wasstretched to 2.5 times its original length.

ethyl acetate, etllefs including P PY and dlbll- With the ratio ofstretch of 1:25 the film has a breaktyl ether and the like. For bestresults, it Was found that ing load of 3700 kilograms per the solventmixture should contain more of the good sol- A strip of thepolycarbonate film, which showed a highvent than the poor solvent. lyoriented crystal diagram upon X-ray examination, was

When casting the film from a polycarbonate solution, wrapped around theconductor of a high voltage cable. the process should be carried out inan atmosphere as dry as possible to prevent the film from becomingopaque. EXAMPLE 3 The stretching Of the polycarbonate film can beeffected Another film from the same polycarbonate of Examy theconventional methods, bowel/6f, it must be ple 2 having a thickness of0.03 mm. was stretched at a Stretch d Wh it is at a temperature Withinthe range of ratio of 1:7.3 under the same conditions. The film had afrom about 100 C. to about 250 C. and more preferbreaking load of 4330kilograms per cm. and an elonably Whil th film is at a temperatureWithin a range gation at break of 28 percent. The film stretched in thisof fr0m about 1 C- t a lll The Stretching manner showed a highlyoriented crystal diagram upon ratio depends on the polycarbonate and isgenerally be- X-ray examination. A strip of this film was used inwraptween 1:2 and 1:12, preferably between 1:3 and 1:5. ping h conductorofahigh voltage cabl In determining the ratio of the degree ofcrystalliza 30 The following table illustrates the effect of temperaturetion of the amorphous material of the polycarbona e, he on thedielectric constant, the insulation resistance and method disclosed byA. Prietzschk, Kolloidzeitschrift 156 loss angle of a partiallycrystallized polycarbonate film. (1958), page 8, may be employed. Inaccordance with TABLE I that method, the rat1o of the degree ofcrystallization to o the amorphous material of the polycarbonate isdivided Temperamrem O 100 by the reflex breadth and the resulting numberis mul- %os?Atn gleO(tgBtX1 50cd/sec s 5 4 tlplled by ten to obtam thecharacterlstlc value The niriialifie iiilffiffil.912911;: 3 .1 31 1:i'lti c: 31 3mm.

formula for this calculation is as follows:

The data depicted in Table I illustrates the dielectric constant,insulation resistance and the loss angle of a partially crystallizedpolycarbonate as a function of tem- Crystalline/amorphous 40 Reflexbreadth X 10 1s equal to the characteristic value perature. Theinsulating quality of the polycarbonate is directly related to itsdielectric strength and inversely re- The molecular weight of thepolycarbonates to be used lated to its loss factor. according to theinvention are in the same range as those In addition, the dielectricconstants of the partially crysof all film forming polymers.Polycarbonates with molecutallized polycarbonate film are generallyclose to the dieleclar Weights from 100 to 150,000, particularly from200 tric constants of the common liquid dielectric materials, to 100,000are preferred. so that the resultant film is considerably more homogene-The thickness of the film employed herein is not critious than otheroil-polymeric insulating materials. However, it is Preferred that the fim have a th ck- Table II compares the physical properties of a partiallyness of from about .01 to about 0.5 mm. and more prefcrystallizedpolycarbonate with that of a noncrystalline erable from about 0.01 toabout 0.1 mm. In preparing the polycarbonate.

TABLE II Loss Angle, Dielectric Tensile Modulus of Polycarbonate tg 6x10Constant, Strength, Elasticity,

Poly[2,2-bis (4-hydroxyphenyl) carbonate] crystallized andlongitudinally stretched film 8 2. 74 22-28 350, 000

Noncrystalline casting film 25 3. 1 8-9 220, 000

layers of the cable, film having different thickness may Analysis of thedata of Table II shows that noncrystalbe used. line polycarbonate has aloss angle more than three times The present invention is furtherdisclosed in the following examples which are illustrative but notlimitagreater than a partially crystallized and longitudinally stretchedpolycarbonate film. Also the dielectric constant of tive thereof. thepartially crystallized polycarbonate film is substantially EXAMPLE 1below that of the noncrystalline polycarbonate material.

A series of films were Prepared from a polycarbonate It will be observedthat the tensile strength of a partially which had been prepared byreacting 2,2-bis(4-hydroxycryslalll'lfed l f f t especlally tlloseStretched In a phenylpmpane with phosgflng The process f ll d inlongitudmal direction, is about three times greater than preparing hpolycarbonate i described in Canadian the noncrystalline polycarbonateand that the modulus Patent 578,585. A solution of this polycarbonatewas preof elasticity of a Partially crystallized Polycarbonate is paredby dissolving about 20% by weight in a suitable more than 1.5 times thatof the noncrystalline polycarsolvent in the unstretched film obtained bycasting the bonate. Also the partially crystallized polycarbonate has afurther advantage, in that the dielectric value is close to thedielectric values of most insulating fluids.

Even though the present invention provides for a polycarbonate filmhaving from 20 to 60 percent crystalline structure, any partiallycrystallized polycarbonate film is adaptable as insulating material forcables.

Various types of insulating oils may be employed in the high voltagecables of this invention. Generally, these insulating oils are eitherhydrocarbon oils isolated from petroleum distillates, such as,naphthenic-base mineral oil, or they are prepared from syntheticdielectric fluids, such as the silicon oils or organosilicon fluids.When a hydrocarbon oil is used, it may be blended with a purified rosinor with a high molecular weight polymer. The unblended naphthenic-basemineral oils are preferred, since they are more resistant todecomposition and have better dielectric properties than the blendedoils.

Satisfactory results have been obtained by using a synthetic siliconefluid, such as polysiloxanes, polysilanes, and polysilicate esters, allof which have excellent dielectric properties.

Although the invention has been described in considerable detail for thepurpose of illustration, it is to be understood that variations may bemade therein by those skilled in the art Without departing from thespirit of the invention and the scope of the claims.

What is claimed is:

1. An electric power cable comprising a conductor SUI-1 rounded by adielectric coating of a partially crystallized polycarbonate film, saidpartially crystallized polycar- 8 bonate film having a crystallinecontent of at least percent.

2. The dielectric coating of claim 1 wherein the polycarbonate film ispoly[2,2-bis(4-hydroxyphenyl)alkyl carbonate].

3. The dielectric coating of claim 1 wherein the polycarbonate has acrystalline content of from to percent.

4. The dielectric coating of claim 1 wherein the polycarbonate is basedon 2,2-bis(4-hydroxyphenyl) propane.

References Cited UNITED STATES PATENTS 2,556,295 6/1951 Pace 264-2,789,967 4/1957 Reynolds et a1 26077.5 3,105,872 10/ 1963 Thompson etal. 174-120 3,214,500 10/1965 Maerov et a1. 26047 3,327,033 6/1967 Kochet 'al 26483 FOREIGN PATENTS 772,627 4/ 1957 Great Britain. 870,095 6/1961 Great Britain.

EARL M. BERGERT, Primary Examiner.

T. R. SAVOIE, Assistant Examiner.

US. Cl. X.R.

1. AN ELECTRIC POWER CABLE COMPRISING A CONDUCTOR SURROUNDED BY A DIELECTRIC COATING OF A PARTIALLY CRYSRALLIZED POLYCARBONATE FILM, SAID PARTIALLY CRYSTALLIZED POLYCARBONATE FILM HAVING A CRYSTALLINE CONTENT OF AT LEAST 20 PERCENT. 